Toothed belts are made of rubber and can have a tensile core wire embedded therein.
The toothed belt can provide a positive drive force without the slipping which can occur with a flat belt or a V-belt. Since the toothed belt also has an advantage of not requiring lubrication, as is required of a gear or a chain drive, its demand and utilization has been greatly increased. The toothed belts are frequently employed in positive drives of overhead cam (OHC) engines for automobiles and extend between pulleys on multiple shafts.
In an OHC engine, the toothed belts are used under severe conditions, e.g. high load and elevated temperature. Therefore, the belt is subjected to flex fatigue and elongation. When the toothed belt used under such severe conditions is elongated about 0.1% or more, the engagement of the belt with the pulleys is deteriorated so that the belt has a tendency to jump off of a pulley with undesirable consequences.
The toothed belt is affected by the characteristics, i.e., flex fatigue and elongation, of the core wire in the belt.
Glass fiber cords each having a high strength, which contributes to a reduction in flex fatigue, and a small elongation have been employed as tensile core wires of a conventional toothed belt. Belts used in an OHC engine for an automobile can have a glass fiber cord which normally is an ECG 150 3/11 or 3/13 having 7.0 to 10.0 times/10 cm upper twists. ECG identifies an electrical (nonalkaline) glass that is a continuous filament and that a single filament has a diameter of about 9 microns. The "150" indicate that a basic filament represents 15,000 yd/ld. Both of the "3"s indicate that 3 filaments are gathered together and first or lower-twisted to form a rope. The "11" and "13" indicate the number of ropes gathered together and final upper-twisted to form the cord. For example, a toothed belt which uses hydrogenated nitrile rubber and has a glass fiber cord having 7 to 10 times/10 cm lower twists and 7 to 9 times/10cm upper twists is disclosed in Japanese Patent Laid-Open No. 62-159827.
However, when the toothed belt is fed under a high load and around small diameter pulleys on multiple shafts in the OHC engine, the glass fiber cord disclosed in this Japanese Patent is elongated, and the tension on the belt is lowered. There are various causes of the elongation and lowering of the tension. It has been discovered that one of the causes is related to the cord configuration and particularly the number of lower twists.
Belts were tested by being run on an apparatus having multiple shafts with each shaft having a pulley and the belt extending between the pulleys. A first toothed belt, having the rubber teeth and the rubber back made of a rubber composition of hydrogenated nitrile rubber and chlorosulfonated polyethylene, a glass core wire having 7 to 10 times/10 cm lower twists, and the rubber teeth covered with canvas, was contracted during its running so that the tension of the belt was raised. Analysis of the belt after running indicated the wear of the canvas at the roots of the teeth was accelerated, and the dedenda of the teeth were visibly cracked. A second toothed belt made of the same rubber composition, a glass core wire having 14 to 18 times/10 cm lower twists and canvas was elongated during its feeding so that the tension of the belt was lowered. After being run under the same conditions of the first belt, analysis of the second belt indicated that the dedenda of the teeth of the second belt cracked early.
An object of this invention is to provide a toothed belt in which glass fiber cords formed of a cord configuration providing low elongation and flex fatigue are employed as core wires and variation in the tension of the belt during running is reduced to provide flex fatigue resistance.